Bowley himself has said that his watch could be 5 minutes off of standard time, which would be expected uncertainty for the average Joe's watch in 1963.
Simulcast transmissions broadcast on both DPD channels show that channel one time and channel two time are within one minute of each other. the "12:30 KKB" transmission on channel two lies between Cussy's "approaching triple underpass" and "go to the hospital" transmissions, placing the assassination at 12:30 channel two time. The McIntire photo shows the Hertz clock displaying "12:30" as the limousine passes the lead car between the TP and the I335E ramp. The watches of Kellerman, Dave Powers, and Sorrels also put the assassination at 12:30. From this, channel one is within one minute of channel two, and channel two is within one minute of standard time.
That doesn't follow. We don't know the accuracy of the Hertz clock or the other watches any more than we know the accuracy of Bowley's watch, Markham's clock, or the clock at Methodist Hospital. As for the "simulcasts", there are two problems. The transcripts of the two channels in CE 1974 are not word-for-word the same for these alleged simulcasts, and even if they were, we still only have the time check for one of the dispatchers at that time, not the other. Furthermore. the recordings were not continuous, have been spliced and edited, and the recording devices were known to be prone to both forward and backward skips, so you cannot even guarantee the order that two separate announcements were made in real time.
